In a typical cellular system, also referred to as a wireless communications network, communication network, Public Land Mobile Network (PLMN) or network, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate with a radio network node in a Radio Access Networks (RAN). The UE may be a mobile terminal by which a subscriber may access services offered by an operator's Core Network (CN) and services outside operator's network to which the operator's RAN and CN provide access. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called an evolved NodeB (eNB), eNode-B, B node or base station. The base station communicates over the air interface operating on radio frequencies with the user equipment(s) within range of the base station. The RAN may implement different Radio Access Technologies (RAT), such as Long Term Evolution (LTE), Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), (Worldwide Interoperability for Microwave Access (WiMAX) etc.
In a roaming situation a UE which has a subscription to an operator of a home public land mobile network (H-PLMN) is served by a visited public land mobile network (V-PLMN). If the UE would like to use a service, like a Voice over IMS service, in the V-PLMN, it may send a request to a control node in the V-PLMN. The control node has to forward this request to a packet data network (PDN) gateway which connects the public land mobile network to a service providing network, which may be an operator Internet Protocol (IP) service network such as IP Multimedia Subsystem (IMS). The control node may select a PDN gateway (PDN GW) based on a roaming agreement between the operator of the V-PLMN and the operator of the H-PLMN for this specific UE. Further the control node may have some further indications to select the PDN GW.
3GPP Standard TS 23.401 V13.1.0 refers in chapter 4.3.8.1 to the PDN GW selection function. According to this standard the PDN GW selection function allocates a PDN GW that shall provide the PDN connectivity for the 3GPP access. The function uses subscriber information provided by the Home Subscriber Server (HSS) in the H-PLMN. The PDN subscription contexts provided by the HSS contain at least the identity of a PDN GW and an Access Point Name (APN) or an APN and an indication for this APN whether the allocation of a PDN GW from this V-PLMN is allowed. If the HSS provides the PDN subscription context that allows for allocation of a PDN GW from the V-PLMN and, optionally, the control node in the V-PLMN is configured to know that the V-PLMN has a suitable roaming agreement with the H-PLMN of the UE, the PDN GW selection function derives a PDN GW identity from the V-PLMN. If a PDN GW identity cannot be derived or if the subscription does not allow for allocation of a PDN GW from the V-PLMN then the APN is used to derive a PDN GW identity from the H-PLMN.
Standard GSMA PRD IR.92 V 8.0 has specified that for a roaming UE which is served by a V-PLMN and has a subscription to an H-PLMN, only a roaming model with a PDN GW in the V-PLMN applies. This roaming model is described in GSMA PRD IR.65 V 8.0 and GSMA PRD IR.88 V 9.0. Hence the control node of the V-PLMN needs to know that local breakout is allowed for a roaming UE in order to be able to set the IMS voice over PS Session Support Indication. The GSMA PRD IR.92 standard has specified that the HSS in the H-PLMN sets a “V-PLMN Address Allowed” (VPAA) parameter when enabling IMS roaming with the V-PLMN wherein the particular subscriber is an IMS subscriber that requires a PDN GW in the V-PLMN as described in GSMA PRD IR.88.
Hence home operator can use the VPAA to indicate that the subscriber is an IMS subscriber authorized for VoLTE roaming by allowing the V-PLMN to select a PDN GW in the V-PLMN and thereby allowing the V-PLMN to send the IMS voice over PS Session Support Indication for indicating that the UE can expect successful execution of the VoLTE service. It is also possible that the home operator can use the VPAA to indicate that the IMS subscriber is not authorized for VoLTE roaming by denying the V-PLMN to select a PDN GW in the V-PLMN and thereby prevent the V-PLMN from sending the IMS voice of PS Session Support Indication. The VPAA is the only indication that H-PLMN can use to steer the behavior of the V-PLMN, while still allowing the UE to connect to the IMS APN. If the VPAA which is received by the control node of the V-PLMN from the HSS in the H-PLMN is set to “allowed” then the control node does not know if a PDN GW 105 in the V-PLMN should be selected even if the roaming model uses a PDN GW in the H-PLMN. This concept has been named as S8HR (S8 Home routed). If the VPAA is set to “not allowed” the control node must select a PDN GW in the H-PLMN. In this case there is no indication from the H-PLMN if the UE has subscribed to an IMS roaming service and can expect a successful IMS Voice over PS session. I.e. there is no indication from the H-PLMN if the control node in the V-PLMN shall set the Voice over PS Session Support Indication to the UE. This may result in that the control node indicates Voice over PS Session Support to the UE even if the subscriber is not an IMS subscriber for voice and the UE would establish unnecessarily a PDN connection and try to register in the IMS. It may also be possible that this may result in that a roaming UE has a roaming subscription and may not receive the Voice over PS Session Support Indication and will hence not use VoLTE when roaming.